Composite structures have been known in the art for many years. Although composite structures can be formed in many different manners, one advantageous technique for forming composite structures is a, automated material placement process, such as an automated fiber placement or automated collation process described in U.S. patent application Ser. No. 10/068,735, filed on Feb. 6, 2002, entitled “Composite Material Collation Machine and Associated Method for High Rate Collation of Composite Materials”. The contents of U.S. patent application Ser. No. 10/068,735 are incorporated herein by reference in its entirety as if fully set forth herein.
In an automated collation technique, one or more ribbons of composite material (also known as composite strands or tows) are laid down on a substrate with a material placement machine. The substrate may be a tool or mandrel, but, can also be formed of one or more underlying layers of composite material that have been previously laid down and compacted.
Fiber placement processes typically utilize a heat source to assist in compaction of the plies of composite material at a localized nip point. In particular, the ribbon or tow of composite material and the underlying substrate are heated at the nip point to increase the tack of the resin of the plies while being subjected to compressive forces to ensure adhesion to the substrate. To complete the part, additional strips of composite material can be applied in a side-by-side manner to form layers and can be subjected to localized heat and pressure during the consolidation process.
Unfortunately, defects can occur during the placement of the composite strips onto the underlying composite structure. Such defects can include tow gaps, overlaps, dropped tows, puckers (i.e., raised regions in a tow), and twists. In addition, foreign objects and debris (FOD), such as resin balls and fuzz balls, can accumulate on a surface of the composite structure which must be detected, identified and eventually removed from the ply surface.
Composite structures fabricated by automated material placement methods typically have specific maximum allowable size requirements for each flaw, with these requirements being established by the production program. Production programs also typically set well-defined accept/reject criteria for maximum allowable number of (i.e., density) of defects-per-unit area and maximum allowable cumulative defect width-per-unit area.
To ensure that the composite laminates fabricated by fiber placement processes satisfy the requirements pertaining to defect size, the structures are typically subjected to a 100% ply-by-ply visual inspection. These inspections are traditionally performed manually during which time the fiber placement machine is stopped and the process of laying materials halted until the inspection and subsequent repairs, if any, are completed. In the meantime, the fabrication process has been disadvantageously slowed by the manual inspection process and machine downtime associated therewith.
Recently, systems and methods have been developed that are capable of detecting, measuring, and marking individual defects in the composite structure. Exemplary systems and methods capable of accurately and reliably detecting, measuring and/or marking defects in a composite structure are disclosed in U.S. patent application Ser. No. 09/819,922, filed Mar. 28, 2001, entitled “System and Method for Identifying Defects in a Composite Structure”; U.S. patent application Ser. No. 10/217,805, filed Aug. 13, 2002, entitled “System for Identifying Defects in a Composite Structure”; and U.S. patent application Ser. No. 10/628,691, filed Jul. 28, 2003, entitled “Systems and Methods for Identifying Foreign Objects and Debris (FOD) and Defects During Fabrication of a Composite Structure.” The entire disclosures of U.S. patent application Ser. Nos. 09/819,922, 10/217,805, and 10/628,691 are each incorporated herein by reference as if fully set forth herein.
Systems and methods have also been developed which are capable of determining a defect characteristic representative of the composite structure, such as a defect density-per-unit area and/or cumulative defect width-per-unit area. Exemplary systems and methods capable of determining defect characteristics are disclosed in U.S. patent application Ser. No. 10/726,099, filed Dec. 2, 2003, entitled “Systems and Methods for Determining Defect Characteristics of a Composite Structure”, the entire contents of which are incorporated herein by reference as if fully set forth herein.
Systems and methods have also been developed which enable a material placement machine to automatically return to defects for manual defect repair, and/or that enable the machine to automatically return to and repair defects without operator intervention. Exemplary systems and methods are disclosed in U.S. patent application Ser. No. 10/799,306, filed Mar. 12, 2004, entitled “Systems and Methods Enabling Automated Return to and/or Repair of Defects with a Material Placement Machine”, the entire contents of which are incorporated herein by reference as if fully set forth herein.
The above-mentioned inspection systems and methods have worked well for their intended purposes and have reduced unproductive down time associated with inspection and repair of laminate plies. The inventors hereof have recognized, however, that such systems and methods can be even further improved by providing suitable non-contact marking alternatives to ink-based surface marking systems. By way of background, ink-based marking systems can employ various means, such as inkjet marking, pump-fed felt-tip marker, spring-loaded marking pen, among others, to deposit an amount of ink onto the composite structure in those areas where defects have been detected. With these ink-based marking systems, the ink should be carefully selected to ensure compatibility with the composite substrate and to ensure that the ink doesn't contaminate the composite substrate. The ink should also be low enough in viscosity so that it flows freely through the supply lines and the sprayer nozzle. The solvents used in the ink should also allow the ink to dry quickly enough to prevent runs on the part surface, but slowly enough to eliminate clogging. Carefully selecting an ink satisfying all of the conditions can require significant amounts of time and costs.